CONTEMPORARY ADVANCES IN PHYSICS 581 



experiments in this field; at any rate, many a physicist has contented 

 himself with measuring something which he thought to be proportional 

 to the maximum voltage, without knowing what value to assign to 

 the factor of proportionality. 



The toughest problem of all, in respect of measuring current and 

 voltage — or let me say, in respect of finding something significant to 

 measure — is forced upon us by a form of discharge which Hittorf 

 invented (this is probably a more suitable word than "discovered") 

 in 1884. He wound a wire spirally around a tube containing air at 

 low pressure, connected the spiral across a Leyden jar and the terminals 

 of an induction-coil, and thus sent through the wire a sequence of 

 current-pulses which were highly damped wavetrains; they incited a 

 brilliant glow in the tube. Within the spiral, and therefore pervading 

 the gas in the tube, there was of course a magnetic field parallel to 

 the axis and alternating its direction with the alternations of the 

 pulsing current. There was also a circular electric field due to the 

 varying magnetic field, pointing alternately clockwise and counter- 

 clockwise around the axis. Also there was a varying electric field due 

 to the alternating potential-differences between the windings of the 

 spiral. All of these three must have influenced the mobile ions of the 

 gas! Often, as Thomson was later to stress, the discharge takes the 

 form of a brilliant ring, thus suggesting that it is the second of the 

 fields which dominates. But the problem of their relative responsi- 

 bilities is a subtle and very difficult one; and the "ring discharge" is 

 as troublesome to elucidate by theory, as it is easy to realize in practice. 



I have written thus far as though we were concerned entirely with 

 two sorts of stable conditions: the self-sustaining luminous high- 

 frequency discharge, and the oscillations of ions in a gas where the 

 supply of ions is maintained not by the alternating voltage but by 

 some other agency acting independently. We are concerned with 

 these, and with a third matter as well: the process of "breakdown," 

 the sudden onset of the self-sustaining discharge which occurs when 

 the amplitude of the high-frequency voltage across a gas hitherto 

 tranquil is elevated past a critical value. This critical value or 

 "breakdown-potential" is the most frequently measured of all the 

 measurable qualities of the discharge; though strictly speaking it is 

 scarcely a quality of the discharge, but rather of the tranquil gas of 

 which it marks the oncoming transformation. Presumably it is 

 preceded by an intermediate state, in which the oscillating voltage 

 both displaces the ions in the gas, and gives them energy enough to 

 form others; but of this we as yet know little. 



